Orthogonal Frequency Division Multiplexing (OFDM) is a promising candidate for achieving high data rate transmissions in many communication areas. In OFDM, the available bandwidth is divided into a number of channels so that each channel has a small portion of the total bandwidth and is called a sub-carrier. Orthogonality amongst sub-carriers is achieved by using an Inverse Fast Fourier-Transform (IFFT) process. In addition, sub-carriers are overlapped in a frequency domain, which leads to a spectral efficiency. Each sub-carrier delivers one modulated symbol and all sub-carriers are transmitted at the same time so that high data rate is achieved.
In general, a communication channel may be characterized by frequency selective fading. Under this environment, each sub-carrier experiences different channel distortions. In order to achieve reliable communication, the current characteristics of all sub-carriers should be collected at the receiver and are fed back to the transmitter.
In addition, the type of communication for data transmission is a packet-based and the receiver should know when the packet is started.
Techniques to handle frequency selection fading and to determine the start of a packet are called channel estimation and synchronization, respectively, which are conducted using a preamble known to both sides of transmitter and receiver and added at the beginning of the packet.
The preamble for the OFDM used in the IEEE 802.11 standard is shown in FIG. 1. The parts from A1 to A10 are short training symbols that are all identical, and are to detect the boundary of the packet. CP is a cyclic prefix to protect C1 and C2 from inter-symbol interference. C1 and C2 are OFDM symbols used to estimate all sub-carriers.
The preamble used in the HyperLAN/2 standard is shown in FIG. 2. HyperLAN/2 has been designed with several different preambles that are used for different packet types. A and B represent different waveform and I means the inverted signal of A or B. CP is the cyclic prefix and C is the OFDM symbol.
In these two standards, even though the OFDM symbols modulated with bit sequence such as C1, C2 and C are used for channel estimation, they still have the drawbacks of commonly used OFDM symbols, that is, the high peak to average ratio (PAR), which degrades the system performance significantly. In addition, all preambles are divided into two parts, packet detection/symbol timing and channel estimation, leading to significant overheads.